1. Field of the Invention
This invention relates to paper feed mechanism for data printers or the like incorporating a rewind roll for accumulating the paper after it has been printed, and has particular reference to paper rewind roll drive means for printers in which the paper is advanced to permit viewing the last printed amount and is subsequently retracted to properly position the next printing line at the printing station.
2. Description of the Prior Art
In certain types of data printers, the paper is positioned at a printing station during a printing operation and then, following a printing operation, the paper is advanced to a viewing station where the last amount printed can be viewed. Such advancement also includes the usual line spacing increment. Thereafter, as an incident to a new cycle of operation of the printer, the paper is retracted to return the last printed amount directly above the printing station to locate the next printing line at the printing station so that a new amount may be printed directly below the previously printed amount.
Such printers present a problem in driving a rewind roll for accumulating the paper after it is passed through the printing station. This is due to the fact that the rewind roll gradually increases in diameter and therefore the angular rotation of the rewind roll does not bear a direct relation to the linear movement of the paper through the printer. Further, certain printers, as exemplified by that disclosed in my copending U.S. patent application Ser. No. 491,871, filed on July 25, 1974, now U.S. Pat. No. 3,967,550 incorporate paper line feed mechanisms selectively capable of advancing the paper one, two or more increments as an incident to each printing operation. Thus, the paper rewind mechanism must be capable of accumulating the paper when fed in such different increments.
In addition, the paper might be manually advanced different amounts between cycles of operation. Also, for some reason, as when initially threading the paper through the printer, it may be left with an appreciable slack portion unaccumulated on the rewind roll.
Prior rewind mechanisms of the above type generally incorporated a yieldable drive mechanism, such as a slip clutch, for rotating the rewind roll. Although such yieldable drive mechanisms are generally satisfactory, they must provide sufficient driving force to adequately rotate the rewind roll when the paper wound thereon is of maximum diameter, i.e. when the moment arm formed by the radius of the rewind paper roll is greatest. However, when only a small amount of paper is accumulated on such rewind roll, i.e. when the moment arm formed by the radius of the paper is the smallest, the same tension is applied to the paper, and in extreme cases, this force may be sufficient to pull the paper through the normal paper feed mechanism or it could conceivably tear the paper. In any event, such prior rewind drive mechanisms tend to apply undue strain to the drive mechanism and to the guide means for the paper. This condition is aggrevated by the increased mass of the paper as it is accumulated on the rewind roll.